Heat treatment and alloy



United States Patent 3,235,415 HEAT TREATMENT AND ALLUY Adolph Edward Palty, Lynnlield, and Paul Adolph Bergman, Nahant, Mass, assignors to General Electric Company, a corporation of New Yorlr No Drawing. Filed Dec. 28, 1961, Ser. No. 162,992 9 Claims. (61. 148-125) This invention relates to the heat treatment of iron base alloys and, in particular, to an improved heat treatment for those iron base alloys which can be transformed from an austenitic structure to a martensitic or partially martensitic structure, and to the alloy structure formed.

in general, the heat treatment of iron base alloys involves subjecting the alloy to a series of temperature conditions for times suflicient to dissolve phases, form precipitates, relieve stresses, change structures, grow grains and the like. it is well known to perform such individual steps singly or in certain combinations. However, the heat treatment of the group of iron base alloys which can be transformed from an austenitic to a martensitic condition requires special understanding in order to achieve a combination of strength and ductility along with suitable corrosion resistance required for the manufacture of articles.

Prior to the present invention, the iron base alloys of this type which were most suitable for use as highly stressed articles, such as in high temperature operating power producing apparatus, were susceptible to corrosion at the strength levels required. For example, one material widely used as blading for gas turbines, and sometimes referred to as AlSI 403 stainless steel, will rust under corrosive conditions within 2 to 20 hours.

The principal object of this invention is to provide a heat treatment method which, through microstructure control, imparts tailored combinations of corrosion resistance and strength to articles made from iron base alloys transformable from an austenitic to a martensitic condition.

Another object of this invention is to provide a double transformation heat treatment which results in the reduction of grain boundary chromium-rich phases and of the corrosion sensitivity tempering range.

These and other objects and advantages will be more readily recognized from the following detailed description and examples which are exemplary of rather than limitations on the scope of this invention.

Briefly, the method aspect of this invention in one form comprises heating an iron base alloy to a first ten"- perature at which substantially complete or full solutioning occurs, cooling at a rate suflicient to inhibit formation of grain boundary phases to a second temperature which is below that (M at which martensite starts to form (and preferably approaches that temperature at which martensite transformation is substantially complete) to produce as a by-product of the martensite transformation numerous and well distributed nucleation sites. The method in another form then continues with the heating to a third temperature lower than the first temperature but within the austenitic range, holding at that temperature to form as such nucleation sites uniform precipitation products predominating in the matrix rather than at the grain boundaries, and then cooling to and holding at a fourth temperature which is below that (M at which martensite starts to form (and preferably approaching that temperature at which martensite transformation is substantially complete) to transform the alloy structure to martensite. The final step of heating to and holding at a fifth temperature to provide desired physical, chemical and mechanical properties can then be con-ducted.

A material typical of the type with which the method of this invention can be practiced and sometimes called ice a controlled transformation stainless steel," semi-ans tenitic or precipitation hardening type stainless steel, has a composition, by weight, of about 0.l0.2% C, LS-1.5% Mn, 1517% Cr, 24% Mo, 45% Ni, with the balance essentially iron and impurities.

It has been the practice to heat treat this material in a variety of ways according to known practices in the art. However, at the higher strength levels suitable for use as gas turbine blades, corrosion resistance is poor with such known heat treatments. When the heat treatment has been changed to improve corrosion resistance, the strength level becomes too low.

in the annealed condition, this material has forming properties similar to an austenitic stainless steel. However, unlike such steels, it can be hardened by martensitic transformation to result in high strength at temperatures below 1000" F.

In the manufacture of certain articles from thl material, it is desirable to place the material in a condition of maximum ductility minimum strength for forming operations at lower temperatures. The particular above identified material can be placed in this condition by heating to ()2l00 F. to dissolve most carbides and impact maximum austenite stability at low forming temperatures.

Following this, heat treatments used in common practice to transform this particular material to a predoi inantly martensitic structure entail heating to one or more partial solutioning temperatures between 1360 F. and 1800 F. to promote the precipitation of carbides or similar precipitates, then cooling to a temperature low enough for transformation to a predominantly martensitic structure, for example, generally between l50 F. and room temperature. Then it is heated to a temperature in the range of 800 F.l000 F. for the desired combination of physical and mechanical properties. However, once the material has been heated within the range of 1850 F.-210{) F., conventional heat treatments of the type described in this paragraph result in structures that exhibit poor corrosion resistance. The reason for this is that such heat treatments do not include techniques to control the distribution of the precipitation, in this case particularly chrominum-rich carbides at the grain boundaries as well as excessive coherent precipitates.

In the manufacture of certain articles, temperatures above 1850 F. are desirable for other reasons. These reasons include obtaining maximum reduction and close tolerances during working operations such as forging. Again heat treatments of the type described in the preceding paragraph lead to structures with poor corrosion resistance.

Therefore another object of this invention is to use the substantially full solution heat treatment, such as of 18S02l5 0 F. with the above mentioned alloy along with a subsequent novel heat treatment to result in excellent corrosion resistance in addition to good strength and machinabilit".

In contrast to the conventional heat treatment, the heat treatment method of this invention involves a controlled nucleation and precipitation method in order to more carefully control microstructure and to develop an optimum combination of corrosion resistance and mechanical properties. The method of this invention eliminates excessive grain boundary carbides and other phases. It also reduces or eliminates the coherent precipitates which are normally formed in the 800-1000" F. tempering cycle of the conventional heat treatment. hese phenomena associated with conventional heat treatments sensitize the material to corrosion attack.

The various approaches to the problem of achieving different combinations of corrosion resistance and strength are illustrated in Tables I and II which are discussed in detail later in connection with the specific above identified alloy which is typical of its class.

In the following Table I, the heat treatment cycles shown were conducted after solutioning between 1850- 2000 F. In the heat treatments shown in Tables I, II and III, the letter C means cool to between about 100 and l20 F. for a minimum of 3 hours. The times at the temperatures given in the tables are 12 hours for temperatures between l3002000 F. and 23 hours for temperatures between 6501l00 F. although longer periods of time can be used. In Table I under Corrosion Data-Rating, P means poor, G means good and E means excellent." Under Type of Attack the letters IGC means intergranular cracking of the stress corrosion specimen, IGA means intergranular attack, none under I-Iours to Begin Attack means no attack in 1000 hours except in Example 19 Where it means no attack in 2000 hours. The corrosion tests were comparisons in a standard ASTM 5% aqueous NaCl solution spray chamber at 95 F.

TAB LE I The heat treatments of Examples 2 and 6 in Table I represent conventional practice. The final cycle of these conventional heat treatments is a 1000 F. tempering operation. Although some heat treatments used in the industry have included an 850 F. tempering cycle as shown in Examples 1 and 5, the l000 F. tempering cycle is specifically preferred in the practice of this invention because it yields a better combination of ductility, fatigue strength, impact strength, stress-corrosion resistance and machinability. The conventional heat treatments with either the 1000 F. or the 850 F. tempering cycles are subject to intergranular corrosion attack. Therefore, both the 850 F. and the 1000 F. tempering temperatures are within the tempering temperature corrosion sensitivity range which exists as a result of the conventional heat treatments.

Tempering outside of the corrosion sensitivity range can offer satisfactory corrosion resistance for some relatively low temperature, relatively low stress applications. For example, the tempering range effects on corrosion are Corrosion Data Example Heat Treatment 1".)

' Attack Hours to Begin Attack In Table II room temperature tensile strength properties are compared with heat treatment, the strength data being give in 1000 p.s.i. units. Although a solution temshown in Examples 3, 4 and 7 of Table I for the tempering temperatures of 550 F., 650 F. and ll0O F. However, if the operating temperature of the article thus heat perature of 2000 F. is shown in the examples of Table treated is above 650 F. and, for example, in the temper- II, it has been found that strength increases as the solution ing temperature sensitivity range, operation of the part temperature is decreased toward l850 F. In Table II, will temper the material and bring about attack on the UTS means ltimate tensile strength, YS" means article. On the other hand tern erinr as hi h as l I p J g yield strength (0.2 or 0.02% offset), percent E means F. can, as shown in Table II, Example 28, result in a percent elongation as percent inches per inch in a 1 inch 5 significant decay in strength. Therefore, even though the gage length and percent R. means percent reduction tempering at 550 F., 650 F. and 1100 F. is shown in area. to have excellent corrosion resistance, nevertheless for TABLE II Ex. Heat Treatment; F.) U'IS 0 2% 0.02% Percent- Percent Ys YS E RA 2,0u0+1,425+1,750+0+650 r. 219 167 15 5s 2,os0+1,425+1,75s+0+s50 229 193 159 15 54 2,00t)l-C'i-1,75il+(la-1,000 191 178 157 1s 09 some applications operating under relatively high stress or at temperatures within the tempering sensitivity range or both, these tempering temperatures cannot be used effectively.

The corrosion sensitive range of approximately 800 1000 F. coincides with the secondary hardening range as illustrated in Table II by comparing the strength properties of Examples 1 and 2 with Examples 25 and 28. It is understood that the secondary hardening range also coincides with the formation of coherent precipitates in the alloy. This is recognized from the fact that with a high degree of secondary hardening at 850 F. temper in Example 1 of Table H, the corrosion resistance is so poor as to lead to stress corrosion cracking as shown by Examples 1 and 3 in Table I. Thus the corrosion sensitivity can be partially attributed to the eflects of the coherent precipitates. Tempering outside of the tempering sensitivity range can offer a solution in only some corrosion applications, provided that the deviations in strength, ductility and dimensional stability are acceptable. In applications such as highly stressed blading in turbomachinery, such deviations cannot be tolerated and the present invention provides a solution to the corrosion problem.

Another approach to a solution of the problem consists in the variation of the partial solutioning cycle, shown by Examples 9 and 10 in Table I. The partial solution temperatures of 1425 F. and 1700 F. resulted in intergranular attack, the 1425 F. treatment having the least corrosion resistance of the two in that it evidenced stress corrosion cracking. The effect of a 550 F. tempering cycle was investigated with the 1425 F. partial solution treatment as shown in Example 8. This treatment resulted in stress corrosion cracking even though the tem pering cycle was outside the secondary hardening range. Photomicrographs show a heavy grain boundary precipitate, presumably chromium-rich, which occurs in the partial solutioning temperature range. This indicates that elimination of the grain boundary precipitate might eliminate the intergranular attack.

To this effect, a simple heat treatment including full solutioning was investigated as shown in Examples 11, 12 and 13. Metallographic examinations of these examples show the elimination of the heavy grain boundary precipitate but also show that this treatment led to severe pitting attack in corrosion with a 1000 F. tempering cycle. On the other hand, tempering temperatures of 550 F. and 1050 F. had excellent corrosion resistance. This demonstrates that in this type of heat treatment, the coherent precipitates (secondary hardening) leads to corrosion independently of heavy grain boundary precipitates.

It was unexpectedly recognized that the method of the present invention results in a new alloy structure which is consistent with operating requirements. The structure which results from the method of this invention has grain boundaries substantially free of heavy or continuous precipitates or both with a reduction or elimination of the effects of the coherent precipitates in both the matrix and the grain boundaries.

The method of the present invention in one form achieves this desirable structure according to the following general steps:

First, a full or a high degree of partial solutioning is needed;

Second, numerous and well distributed nucleation sites must be formed in a manner to attain the objectives of the remaining steps;

Third, at the nucleation sites uniform precipitation is induced which predominates in the matrix rather than at the grain boundaries to substantially eliminate heavy grain boundary precipitates and control the availability of the elements which can enter into coherent precipitates.

Subsequent steps to achieve a tempered martensitic structure are carried out in a conventional manner.

The novel method aspect of this invention can be implemented by heat treatment forms as described hereinafter:

In the first step, substantial solutioning of grain boundary precipitates is achieved at temperatures between about 1850 F. to 2150 F. shorter periods of time being required for the higher temperatures. For example, at 1950 F. about 15-20 minutes time at temperature is preferred. Furthermore, coherent precipitates are dissolved and incoherent matrix precipitates are substantially or partially dissolved depending on temperature and time. The greatest solutioning is obtained by the longest times at the highest temperatures.

In the second step, numerous and well distributed nucleation sites are created as a by-product of martensitic transformation. It is necessary to cool to about room temperature or below at a sutficiently rapid rate to minimize precipitation particularly at the grain boundaries during cooling. Sub-zero Fahrenheit cooling for at least about 1 hour and generally between 3 and 20 hours is preferred in the practice of the present invention to achieve substantial martensitic transformation particularly with the alloy composition identified above.

In the third step, at the nucleation sites previously formed, uniform precipitation is induced predominantly in the matrix rather than at the grain boundaries at partial solutioning temperatures between 1350 F. and a temperature slightly below 1850 F., for at least about 1 hour and preferably between 2 and 20 hours, for example 1 hour at 1425 r. in general the lower the partial solution temperature, the greater is the amount of precipitation formed and the less is the amount available of those elements which eventually can enter into coherent precipitates. This, then, leads to greater corrosion resistance but lower strength. Conversely, the higher the partial solutioning temperature, the lower is the corrosion resistance and the higher is the strength.

The final step includes cooling from the partial solutioning temperature to a temperature low enough to insure substantially complete transformation to martensite for at least about 1 hour, such as by first cooling to about room temperature and then for a minimum of about 3 hours. For example, this step is preferred to be conducted between and F. for about 12-20 hours.

A subsequent tempering operation can be conducted to obtain the combination of properties desired, such as physical, chemical and mechanical properties, for example at least about 1 hour and preferably about 2 hours at between 550 and 1100 F.

The heat treatment forms which exemplify the concepts and principles of the present invention are shown by examples 1520 in Table I. When a partial solution treatment at 14-25 F. is used, the corrosion resistance is excellent independently of tempering temperatures in the range of 650 F. to 1050 F. It is to be noted in these same examples in Table II that secondary hardening effects have been greatly reduced or eliminated. A 1050 F. tempering temperature leads to some over-aging and a slight reduction in strength.

The corrosion resistance and strength with other partial solutioning temperatures are shown in Tables I and II. A partial solution temperature of 1300 F. as shown in Example 14 was near the lower limit for precipitation at the nucleation sites for a particular heat of the specific allow identified hereinbefore.

In Example 21, a 1550 F. partial solution temperature proved to have excellent corrosion resistance with a 1000 F. tempering cycle. In Examples 22 and 24, when the partial solution temperatures were increased to 1650 F. and 1700 F. with a 1000 F. tempering cycle, the material became less corrosion resistant. Also there is an increase in tensile strength as the partial solution temperature increases, as shown in Examples 19, 35, 37, 39 and 41 in Table II. This is attributed to higher carbon martensite and the coherent precipitation effects. However, even with the 1700 F. treatment in Example 39, which is comparable with Example 24 of Table I, the corrosion resistance has been greatly improved over the conventional heat treatments. This is shown by the fact that attack takes place as pitting and begins in 400 to 600 hours in standard salt spray testing as compared with severe intergranular attack resulting from conventional heat treatments and which begins in less than 2 hours in such a comparison test. A tempering temperature of 1050 F. results in excellent corrosion resistance even with higher partial solution temperatures as shown in Example 23. However the 1050 F. temper reduces strength as shown in Examples 20, 36, 38 and 40 in Table II. The increased corrosion resistance and dereased strength are due to the over-aging of the coherent precipitates (complete or partial loss of coherency).

Because partial solution temperature increases from 1425 F. 1750 F. result in greater strength, it is apparent that considerably greater strength can be achieved with an 850 F. tempering cycle and the higher partial solution temperatures. This treatment, which takes advantage of the higher-carbon martensite and coherent precipitation, leads to lower corrosive resistance.

It has thus been shown that it is possible according to the present invention to achieve combinations of corrosion resistant and strength which are tailored to the requirements of particular applications. Some examples of such combinations are shown in the following Table III.

Although the present invention has been described in connection with specific examples and particularly with a specific alloy, it will be recognized by those skilled in the art the variations and modifications of which the invention is capable.

What is claimed is:

1. A method of heat treating a controlled transformation stainless steel which can be transformed from an austenitic structure to at least a partially martensitic structure, comprising the steps of:

heating to a first temperature of 185 -2150 F. to bring about substantially complete solutioning of the alloy structure;

cooling at a rapid rate to inhibit formation of grain boundary phases, to a second temperature below about room temperatre for at least about 1 hour to produce martensite and, as a by-product of the martensite transformation, numerous and well distributed nucleation sites predominantly in the matrix; heating to a third temperature of between 1425 F. and slightly below 1850 F. within the austenitic range of the alloy to induce uniform precipitation in the matrix of the alloy strutcure at the nucleation sites; holding at the third temperature for at least about 1 hour to form at the nucleation sites uniform precipitation predominantly in the matrix;

cooling to a fourth temperature below about room temperature for at least about 1 hour to transform the structure of the alloy to martensite; and then,

heating to a fifth temperature between 7501100 F.

for at least about 1 hour to provide desired properties.

2. A method of heat treating a controlled transformation stainless steel which can be transformed from an 8 austenitic structure to at least a partially martensitic structure, which alloy requires cooling at least to about 0 F. to bring about such transformation, comprising the steps of:

heating to a first temperature of between 1850 and 2150 F. to bring about substantially complete solutioning of the alloy structure;

cooling at a rapid rate to inhibit formation of grain boundary phases, to a second temperature below about 0 F. for at least about 1 hour to produce martensite and, as a by-product of the martensite transformation, numerous and well distributed nucleation sites predominantly in the matrix;

heating to a third temperature of between 1425 F.

and slightly below 1650 F. within the austenitic range of the alloy to induce uniform precipitation in the matrix of the alloy structure at the nucleation sites;

holding at the third temperature for at least about 1 hour to form at the nucleation sites uniform precipitation predominantly in the matrix;

cooling to a fourth temperature below about 0 F. for

at least about 1 hour to transform the structure of the alloy to martensite; and then heating at a fifth temperature between 750 and 1100" F. for at least about 1 hour to provide desired properties.

3. A method of heat treating a controlled transformation stainless steel which can be transformed from an austenitic structure to at least a partially martensitic structure, which alloy requires cooling to at least about 0 F. to bring about such transformation and which alloy comprises by weight 01-02% C, 05-15% Mn, 1517% Cr, 24% Mo, 45% Ni, with the balance essentially Fe and impurities, comprising the steps of:

heating at a first temperature of between 1850 and 2150 F. to bring about substantially complete solutioning of the alloy structure;

cooling at a rapid rate to inhibit formation of grain boundary phases, to a second temperature of between about F. and F. for at least 3 hours to produce martensite and, as a by-product of the martensite transformation, numerous and well distributed nucleation sites predominantly in the matrix;

heating to a third temperature between 1425 and 1650 F. within the austenitic range of the alloy to induce uniform precipitation in the matrix of the alloy structure at the nucleation sites;

holding at the third temperature for at least about 1 hour to form at the nucleation sites uniform precipitation predominantly in the matrix;

cooling to a fourth temperature between about 100 F. and 120 F. for at least 3 hours to transform the structure of the alloy to martensite; and then heating at a fifth temperature between 850 and 1050 F. for at least about 1 hour to provide desired properties.

4. A method of heat treating a controlled transformation stainless steel which can be transformed from an austenitic structure to at least a partially martensitic structure, which alloy requires cooling to at least about 0 F. to bring about such transformation and which alloy comprises by weight 01-02% C, 05-15% Mn, 1517% Cr, 24% Mo, 45% Ni, with the balance essentially Fe and impurities, comprising the steps of:

heating to a first temperature of between 1850 and 215 0 F. to bring about substantially complete solutioning of the alloy structure;

cooling at a rapid rate to inhibit formation of grain boundary phases, to a second temperature of between about 100 F. and 120 F. for at least 3 hours to produce martensite and, as a lay-product of the martensite transformation, numerous and well distributed nucleation sites predominantly in the matrix;

heating to a third temperature of about 1425 F. within the austenitic range of the alloy to induce uniform precipitation in the matrix of the alloy structure at the nucleation sites;

holding at the third temperature for at least about 1 hour to form at the nucleation sites uniform precipitation predominantly in the matrix;

cooling to a fourth temperature between about 100 F. and -120 F. for at least 3 hours to transform the structure of the alloy to martensite; and then heating at a fifth temperature of about 1000 F. for at least about 1 hour to provide desired properties.

5. A method of heat treating a controlled transformation stainless steel which can be transformed from an austenitic structure to at least a partially martensitic structure, which alloy requires cooling to at least about F. to bring about such transformation and which alloy comprises by weight 01-02% C, -15% Mn, 15-17% Cr, 24% Mo, 4-5% Ni, with the balance essentially Fe and impurities comprising the steps of:

heating to a first temperature of between 1850 and 2150 F. to bring about substantially complete solutioning of the alloy structure;

cooling at a rapid rate to inhibit formation of grain boundary phases, to a second temperature of between about -l00 F. and 120 F. for at least 3 hours to produce martensite and, as a byproduct of the martensite transformation, numerous and well distributed nucleation sites predominantly in the matrix;

heating to a third temperature of about 1700 F. within the austenitic range of the alloy to induce uniform precipitation in the matrix of the alloy structure at the nucleation sites;

holding at the third temperature for at least about 1 hour to form at the nucleation sites uniform precipitation predominantly in the matrix;

cooling to a fourth temperature between about -100 F. and -120 F. for at least 3 hours to transform the structure of the alloy to martensite; and then heating at a fifth temperature of about 1050 F. for at least about 1 hour to provide desired properties.

6. A method of heat treating a controlled transformation stainless steel which can be transformed from an austenitic structure to at least a partially martensitic structure, which alloy requires cooling to at least about 0 F. to bring about such transformation and which alloy comprises by weight 0.10.2% C, 0.51.5% Mn, 15-17% Cr, 24% Mo, 45% Ni, with the balance essentially Fe and impurities comprising the steps of:

heating to a first temperature of between 1850 and 2150 F. to bring about substantially complete solutioning of the alloy structure;

cooling at a rapid rate to inhibit formation of grain boundary phases, to a second temperature of between about 100 F. and 120 F. for at least 3 hours to produce martensite and, as a by-product of the martensite transformation, numerous and well distributed nucleation sites predominantly in the matrix;

heating to a third temperature of about 1750 F. within the austenitic range of the alloy to induce uniform precipitation in the matrix of the alloy structure at the nucleation sites;

holding at the third temperature for at least about 1 hour to form at the nucleation sites uniform precipitation predominantly in the matrix;

cooling to a fourth temperature between about 100 F. and -120 F. for at least 3 hours to transform the structure of the alloy to martensite; and then heating at a fifth temperature of about F. for at least about 1 hour to provide desired properties.

7. A method of heat treating a controlled transformation stainless steel which can be transformed from an austenitic structure to at least a partially martensitic structure, which alloy requires cooling to at least about 0 F. to bring about such transformation and which alloy comprises by weight 01-02% C, 05-15% Mn, 15-17% Cr, 24% Mo, 4-5 Ni, with the balance essentially Fe and impurities comprising the steps of:

heating to a first temperature of between 1850 and 2150 F. to bring about substantially complete solutioning of the alloy structure;

cooling at a rapid rate to inhibit formation of grain boundary phases, to a second temperature of between about 100 F. and F. for at least 3 hours to produce martensite and, as a by-product of the martensite transformation, numerous and well distributed nucleation sites predominantly in the matrix;

heaing to a third temperature of about 0 F. within the austenitic range of the alloy to induce uniform precipitation in the matrix of the alloy structure at the nucleation sites;

holding at the third temperature for at least about 1 hour to form at the nucleation sites uniform precipitation predominantly in the matrix;

cooling to a fourth temperature between about 100 F. and 120 F. for at least 3 hours to transform the structure of the alloy to martensite; and then heating at a fifth temperature of about 850 F. for at least about 1 hour to provide desired properties.

8. A corrosion resistant steel of the type which can be transformed from an austenitic structure to at least a partially martensitic structure during the heat treatment thereof, and having an internal structural arrangement which includes a combination of (1) small amounts of discontinuous chromium-rich phases at the grain boundaries and (2) an evenly distributed precipitation of discontinuous phases throughout the matrix, the combination reducing the effects of coherent precipitation to substantially eliminate the tempering corrosion sensitivity range.

9. A corrosion resistant controlled transformation stainless steel which can be transformed from an austenitic structure to at least a partially martensitic structure, which alloy requires cooling to at least about OF. to bring about such transformation and which alloy comprises by weight o.1-0.2% c, 05-15% Mn, 15-17% ta, 2-4% Mo, 4-5% Ni, with the balance essentially Fe and impurities, and having an internal structure which includes a combination of (1) small amounts of discontinuous chromium-rich carbides at the grain boundaries and (2) an evently distributed precipitation of discontinuous phases throughout the matrix, the combination reducing the effects of coherent precipitation to substantially eliminate the tempering corrosion sensitivity range.

References Cited by the Examiner UNITED STATES PATENTS 2,395,608 2/1946 Aborn 148-36 2,400,842 5/ 1946 Schaufus 14836 3,046,167 7/1962 Waxweiler et al. 148135 FOREIGN PATENTS 585,615 10/1959 Canada.

DAVID L. RECK, Primary Examiner.

RAY K. WINDHAM, Examiner. 

1. A METHOD OF HEAT TREATING A CONTROLLED TRANSFORMATION STAINLESS STEEL WHICH CAN BE TRANSFORMED FROM AN AUSTENITIC STRUCTURE TO AT LEAST A PARTIALLY MARTENSITIC STRUCTURE, COMPRISING THE STEPS OF: HEATING TO A FIRST TEMPERATURE OF 1850-2150*F. TO BRING ABOUT SUBSTANTIALLY COMPLETE SOLUTIONING OF THE ALLOY STRUCTURE; COOLING AT A RAPID RATE TO INHIBIT FORMATION OF GRAIN BOUNDARY PHASES, TO A SECOND TEMPERATURE BELOW ABOUT ROOM TEMPERATRE FOR AT LEAST ABOUT 1 HOUR TO PRODUCE MARTENSITE AND, AS A BY-PRODUCT OF THE MARTENSITE TRANSFORMATION, NUMEROUS AND WELL DISTRIBUTED NUCLEATION SITES PREDOMINANTLY IN THE MATRIX; HEATING TO A THIRD TEMPERATURE OF BETWEEN 1425*F. AND SLIGHTLY BELOW 1850*F. WITHIN THE AUSTENITIC RANGE OF THE ALLOY TO INDUCE UNIFORM PRECIPITATION IN THE MATRIX OF THE ALLOY STRUCTURE AT THE NUCLEATION SITES; HOLDING AT THE THIRD TEMPERATURE FOR AT LEAST ABOUT 1 HOUR TO FORM AT THE NUCLEATION SITES UNIFORM PRECIPITATION PREDOMINANTLY IN THE MATRIX; COOLING TO A FOURTH TEMPERATURE BELOW ABOUT ROOM TEMPERATURE FOR AT LEAST ABOUT 1 HOUR TO TRANSFORM THE STRUCTURE OF THE ALLOY TO MARTENSITE; AND THEN, HEATING TO A FIFTH TEMPERATURE BETWEEN 750-1100*F. FOR AT LEAST ABOUT 1 HOUR TO PROVIDE DESIRED PROPERTIES. 